Equalizers or similar devices have been used for adjusting sound quality of digital audio signals. Equalizers adjust the frequency characteristic of input audio signals. Specifically, equalizers achieve desired sound quality adjustment by emphasizing or suppressing a specific frequency range of input audio signals. The audio signals output from the equalizers are converted into analog signals by digital-analog (D/A) converters and are provided to analog signal processors including output devices (such as speakers, headphones, earphones, and receivers for conversation).
When equalizers emphasize a specific frequency range of audio signals, clipping occur in which the amplitude level of audio signals output from the equalizers exceed the maximum amplitude level capable of being operated by D/A converters. Hearers are prone to sense the deterioration of sound quality at the clipping occurring portion.